1. Field of the Invention
The present invention relates to the magnetic resonance imaging (MRI) technology, and particularly to a tubular surface coil and a method and system for processing radio frequency (RF) signals of such a tubular surface coil.
2. Description of the Prior Art
The basic principle of MRI is that hydrogen atoms (or other atoms, but hydrogen atoms are most commonly used) in human tissues will be directionally aligned under the effects of a fixed magnetic field. When an external radio frequency pulse is applied, these hydrogen atoms will be deviated due to the effects of the radio frequency pulses. After the radio frequency pulses vanishes, these hydrogen atoms will recover to the original state. During the recovering process, sampling radio frequency signals generated by these hydrogen atoms and then reconstructing an image using the acquired signals can result in an image of the human tissues. Since the distribution of hydrogen atoms in different tissues varies, different human tissues can be distinguished through the obtained images.
In MRI equipment, a coil is a device for acquiring such signals, the basic principle being similar to that of a radial field receiving antenna. According to the relationship with respect to a human body, such coils can be divided into body coils, and surface coils, etc., and according to the shape, the coils can be divided into: tubular coils, planar coils, helmet-shaped coils, fan-shaped coils, etc. A knee coil is a tubular surface coil, and by taking the knee coil as an example hereinbelow, the structure and imaging features of conventional tubular surface coils will be explained.
FIG. 1 shows a diagram of the construction structure of an existing knee coil. Referring to FIG. 1, wherein:
part (a) is the appearance of a conventional knee coil, which is tubular;
part (b) is a schematic diagram of the exploded structure of the existing knee joint coil; according to the exploded schematic diagram, the construction units of the coil are coil units, and in FIG. 1, the number of coil units is six as an example, and E1 to E6 represent the coil units; and
part (c) is a diagram of positional relationship between the coil units when the six coil units are arranged into a tubular shape.
During the operation of the MRI equipment, each coil unit acquires corresponding radio frequency signals, and the acquired radio frequency signals are vectors and are sent to a system for processing radio frequency signals in the MRI equipment, therefore, these signals sent by the coil to the system for processing radio frequency signals are referred to as coil output signals. The system for processing radio frequency signals is used for performing radio frequency signals processing of the coil output signals which are then sent to an image reconstruction system to reconstruct an image, and FIG. 2 shows a schematic diagram of an existing system 200 for processing radio frequency signals. Referring to FIG. 2, the existing system 200 for processing radio frequency signals comprises: N receiving channels 210 and a radio frequency signal processing module 220.
The N receiving channels 210 are used for receiving N channels of the coil output signals coming from the coil, and sending the N channels of coil output signals to the radio frequency signal processing module 220. N is an integer greater than 1 and less than or equal to the number of coil units in the coil (the number of coil units in the coil is set as M below, with M being an integer greater than 1). If the number N of receiving channels is equal to the number M of coil units in the coil, M channels of radio frequency signals can be taken directly as the coil output signals, and sent to the corresponding N receiving channels, and if the number N of receiving channels is less than the number M of coil units in the coil, N channels of coil output signals can be generated after certain signal synthesizing of M channels of radio frequency signals, and sent to the corresponding N receiving channels.
The radio frequency signal processing module 220 is used for performing radio frequency signal processing on the received coil output signals. In the prior art, said radio frequency signal processing is as follows: summing the square of the modulus values of each of the received channel of coil output signals, and computing the square root of the obtained sum. The result of the square root computation can be used in performing image reconstruction. Taking the coil shown in FIG. 1 as an example, assuming that the number N of receiving channels is equal to the number M of the coil units in the coil, then said radio frequency signal processing is as follows: calculating square (S012+S022+ . . . +S062), wherein “square” represents the square root computation, and where S01, S02, . . . , S06 represent the radio frequency signals from coil units E1, E2 . . . E6, respectively.
The image reconstruction will obtain three types of basic images: lateral cross-sectional plane images, sagittal plane images and coronal plane images, and the images in other sections can also be obtained by performing a certain transformation on said three types of basic images.
The signal strength acquired by the coil units in the surface coil at a close distance is much greater than that acquired at a farther distance. This results, in the images obtained according to the abovementioned method, in both the signal strength between different areas (for example, the surface area and central area of the tubular surface coil) and the signal to noise ratio (SNR) proportional to the signal strength are significantly different, that is, the signal strength and the uniformity of signal to noise ratio within an imaging area are relatively poor.
FIG. 3 and FIG. 4 are computer screenshots, respectively showing diagrams of the distribution of signal to noise ratio of the lateral cross-sectional images and the sagittal plane image of an existing knee coil. Since the distribution of signal to noise ratio of the coronal plane images has the same features as that of the sagittal plane images, a diagram of the signal to noise ratio distribution of the coronal plane image is not shown in this application document. In FIGS. 3 and 4, the signal to noise ratio inside tissues and on the surface of tissues are illustrated in the form of contour lines, and the higher density of curves represents the greater difference. It can be seen from FIGS. 3 and 4 that, in the lateral cross-sectional images and the sagittal section images obtained by using the existing technology, the signal strength on the tissue surface is much greater than that inside the tissue, and accordingly, the signal to noise ratio on the tissue surface of is also much greater than that inside the tissues. In general, both the signal strength and the uniformity of the signal to noise ratio within the imaging area are relatively poor.